The atmosphere in space is hostile for all materials, including adhesives and epoxies. Low-outgassing adhesives and epoxies should be used in space applications. The standard test for detecting outgassing levels is ASTM E595.

Satellites and spacecraft must operate and thrive in an environment that is vastly different from Earth’s. There is no atmosphere to shield them from the sun’s rays or provide a warm blanket. Instead, they must endure extreme heat and cold, unusual forms of ordinary materials and elements, and high vacuum conditions.

Several components in space, such as propulsion subsystems and some experimental gadgets aboard the International Space Station, must tolerate cryogenic temperatures as low as 4°K. The equipment and materials used in spacecraft are subjected to a number of restrictions as a result of these variables.

Additional climatic considerations may influence the design of a spacecraft depending on its destination. There is extremely reactive atomic oxygen in low-Earth orbits, where space stations and most artificial satellites are installed (AO). Particulate radiation (protons, electrons, and heavy ions) can cause problems in geostationary orbits, where communications and weather satellites operate. High temperatures and severe radiation are of great concern in interplanetary areas and deep space, where space probes traverse.

Adhesive-related issues in space

Bonding, potting, encapsulating, coating, sealing, and staking are all examples of how adhesives and other polymers are employed in spacecraft for structural, mechanical, and electronic functions. Because they are inside or sandwiched and protected between the parts they hold together, most adhesives used on spacecraft are not exposed to atomic oxygen or ionizing radiation. However, most adhesives used in space applications are exposed to high vacuum and extreme cold, regardless of whether they are physically exposed to space or not. They must be able to function properly regardless of the environment without causing damage to other parts of the spaceship.

Low-outgassing adhesives can be used to protect optics for space applications. Otherwise, impurities emitted by the glue can cause the lenses to fog and deteriorate.

On spacecraft, adhesives are frequently used near electronic or optical components, where contamination can cause major issues. Many adhesives, for example, can release volatile chemicals that condense and contaminate electronic, optical, and other precise instruments when they outgas.

Outgassed compounds can obstruct circuit continuity by clouding or fogging optical and electro-optical components. Optical components’ reflectance and absorbency can be influenced by even minute amounts of condensed organic material. Contaminant deposits and the resulting loss of performance to mission-critical subsystems can be avoided with precise control of outgassing levels.

Adhesive specifications

Adhesives for space applications must be able to survive high vacuums, extreme cold, and temperature variations. Their ties should be strong enough to last the duration of the assignment. Adhesives exposed to space for lengthy periods of time should be very resistant to radiation and microcracking, as well as atomic oxygen if in low-Earth orbit. This is due to the fact that radiation can make some adhesives brittle, resulting in cracks. Additionally, prolonged exposure to AO can degrade adhesives.

On spacecraft and satellites, low-outgassing epoxies are employed for structural, mechanical, electrical, and adhesive purposes.

Electrical conductivity, optical clarity, thermal conductivity or insulation, low ionic content, and cryogenic serviceability are some of the other application-specific characteristics.

However, all adhesives used in space must meet the outgassing acceptance standards outlined in NASA’s SP-R-0022A, Vacuum Stability Requirements of Polymeric Material for Spacecraft Application. The overall mass lost from a polymer-based adhesive specimen in a vacuum must not exceed 1% of its original mass, and the collected volatile condensable materials must not exceed 0.10 percent of the original specimen mass, according to this specification. According to an update, the total mass lost may exceed 1.0 percent if it can be demonstrated that the excess mass lost was made up of water vapor.

A standard test known as ASTM E595 is used to screen low-outgassing adhesives and polymers for use in space (see sidebar). For adhesives on spacecraft or equipment slated for use in space, passing this outgassing test is an essential — but not usually sufficient — condition. For example, an adhesive that passes NASA’s outgassing requirements may not always be safe for use near cryogenic instruments.

Some adhesives, such as two-part epoxies, can be carefully designed to emit gases at quantities substantially below the ASTM E595 limit. Some adhesives, such as UV-curable adhesives, were previously unable to pass this test. Chemists have recently cracked the code of UV curables and created numerous that pass the test.

Solvents or moisture are used to cure outgassing adhesives. Several pressure-sensitive and contact adhesives, as well as cyanoacrylates, are among them.

It’s worth mentioning that outgassing might vary a lot even within a single glue family. Because of their lower cross-link density, more flexible epoxies are more prone to outgassing than their more rigid counterparts. As a result, individual adhesives should be tested for outgassing rather than as a group. And the only way to know for sure whether a particular glue fits the ASTM E595 minimal outgassing criteria is to test it.

Epoxies for use in the space

By selecting particular epoxies, the combined issue of achieving sufficiently low outgassing levels while providing physical qualities that suit a variety of applications can be easily handled. Epoxies are versatile polymer compounds with outstanding cohesion, chemical resistance, good adhesion to a variety of substrates, and the ability to act at temperatures ranging from cryogenic (4°K) to 550°F.

As previously stated, outgassing varies greatly between epoxies. A variety of one- and two-part epoxies have been particularly designed to meet or surpass NASA’s outgassing requirements. Electrical conductivity, cryogenic serviceability, optical clarity, thermal conductivity or insulation, and resistance to vibrations, thermal shocks, or impacts can all be found in epoxies that have passed ASTM E595.

Epoxies can be developed to have a wide range of mechanical, thermal, optical, electrical, and physical qualities to meet the needs of a specific application. The strength, viscosity, thermal expansion, heat and chemical resistance, electrical and thermal conductivity, and shrinkage of an epoxy can all be affected by the use of inorganic fillers.

Although one-part epoxies do not require mixing, they do require heat to cure or improve their effectiveness. Once mixed, most two-part epoxies cure at room temperature. Heat enhances the cross-link density of a polymer, which reduces outgassing, therefore using a heat cycle after applying an adhesive can help prevent future issues. For example, after curing a two-part adhesive overnight at room temperature, a third to fifth hour or more at 60 to 80°C completes the cure and improves its outgassing qualities.